Sprague Dawley rats were born and reared under either daylight-simulating fluorescent lights or high pressure sodium vapor (HPSV) lamps. The total irradiance was 425 MuW/cm2 under both luminaires. The adrenal weights of both male and female rats housed under the HPSV lights were significantly higher than those reared under the daylight conditions. These results suggest that HPSV lamps, which have emission spectra that are grossly distorted when compared with natural daylight, may induce stress in rats. The significance of this finding to human health is unknown at the present time.